pH dependence of the kinetic parameters for 3-oxo-.DELTA.5-steroid isomerase. Substrate catalysis and inhibition by (3S)-spiro[5.alpha.-androstane-3,2'-oxiran]-17-one

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Abstract
The pH-rate profiles for kcatobsd and (Kcat/KM)obsd at 25.0.degree. C have been measured for 3-oxo-.delta.5-steroid isomerase by using 5-androstene-3,17-dione (2), 5-pregnene-3,20-dione (3), and 5(10)-estrene-3,17-dione (4) as substrates. Results from the nonsticky substrate 4 suggest values for the pK of a catalytically important group on the free enzyme (pKE) of 4.57 and the pK of the same group in the enzyme-substrate complex of 4.74. For the sticky substrates 2 and 3, pKES is ca. 4.75 and 5.5, respectively. Analysis of the (kcat/KM)obsd vs. pH profile for 2 reveals that the intermediate E.S complex decomposes to products at a rate similar to its reversion to E + S. The pH-rate profile for inhibition of the isomerase by (3S)-spiro-[5.alpha.-andorstane-3,2''-oxiran]-17-one (7.beta.) shows values for pKE of 4.75 and PKEI of 4.90. The similarity of the pH-rate profiles for isomerization of 4 and inhibition by 7.beta. suggests that both reactions may be governed by the ionization state of the same carboxyl group of the enzyme.